Imaging optical system

An imaging optical system which images a line image extending in a main scanning direction in a two-dimensional image as an erected image at a unit magnification in the main scanning direction, wherein

• the imaging optical system comprises an erecting unit optical system consisting of a plurality of refractive index profile type optical elements, each having a refractive index profile where the refractive index is reduced toward opposite ends in the main scanning direction from the center thereof, arranged in the main scanning direction, and a condenser optical system which is disposed along the main scanning direction on the light incident side or on the light incident side and the imaging side of the erecting unit optical system and has a power only in a sub-scanning direction, and
• the numerical aperture in the main scanning direction of each of said refractive index profile type optical elements is smaller than the numerical aperture in the sub-scanning direction of the condenser optical system.